A speed reducer is used in a joint of an industrial robot, and accuracy of a trajectory of an arm is greatly dependent on performance of the speed reducer. When the performance of the speed reducer for the industrial robot is deteriorated, appropriate replacement of the speed reducer is of importance. However, when the speed reducer for the industrial robot is replaced, an industrial robot outfitted with the speed reducer for an industrial robot and a production line where the industrial robot is installed must be stopped. Accordingly, in order to ascertain a time to replace the speed reducer for an industrial robot, an appropriate prediction of a failure in the speed reducer for the industrial robot is of great importance.
A technique which has hitherto been known as a technique for predicting occurrence of a failure in the speed reducer for an industrial robot is to take a sample of a lubricant of the speed reducer, magnetically detect a concentration of iron powder, and predict occurrence of a failure in the speed reducer from the thus-detected concentration of iron powder (see; for instance, Patent Document 1).